1. Field of the Invention
This invention relates to a printed circuit board (PCB) and more particularly to a PCB bonding pad which is partially covered with a solder mask layer for directing a surface mount pin upon the bonding pad from a lateral direction.
2. Background of the Relevant Art
PCBs or printed wiring boards (PWBs) are rigid or flexible single, double or multi-layered boards having printed conductors placed upon or within the board material. A PCB is designed to receive separately manufactured electrical components and to interconnect those components in an overall circuit structure. The components or parts comprise integrated or discrete circuits well-known in the semiconductor art.
PCBs include numerous conductors arranged within a layer or layers extending across the PCB insulated material. Most industrial and commercial PCBs are manufactured from rigid copper-clad, epoxy-impregnated glass fiber laminate. The printed conductors can be selectively formed by a subtractive or additive process. Either process usually includes tin, lead-tin and/or gold plating steps. Formed on one or both surfaces near an edge of the PCB is a row of edge-board contacts (often referred to as "fingers"). The fingers are formed at the same time as the printed conductors and component mounting pads and can be gold plated to permit mating and removal of the PCB into and from a receptor without damaging the contact material. The fingers are spaced from one another and slightly elevated from the upper surface of the PCB. Each finger is configured to receive a compliant pin arranged within the receptor. Flexure of the pin allows frictional engagement and electrical contact between the pin and the contact. Many receptors include a plurality of pins arranged in two rows spaced from one another. The spaced rows define a slot into which the edge of the PC board containing fingers can be frictionally inserted.
There are many types of receptors. A receptor can be, for example, a back plane connector on a mother board or a cable connector of flexible conductors. As stated above, many receptors allow non-permanent engagement and connection with a PCB edge. Other receptors are specifically designed to permit permanent connection between each of the compliant pins and the fingers. Regardless of whether permanent connection or non-permanent connection is desired, it is critical that the pins make electrical contact with respective fingers. The pins must therefore be directed or channeled to their respective positions on the upper surface of the fingers before electrical conductivity can be achieved and/or before permanent connection can be achieved.
A receptor which achieves permanent connection to fingers is generally configured having elongated compliant pins spaced from one another and extending in at least one row. The distal end (or "toe") of each pin extends laterally from the edge of the card, flexes upward and onto the surface of the card toward a forward facing edge of the finger. The fingers, extending above the PCB surface, forces further upward compliance of the pin such that the toe of each pin resides upon the finger tipper surface. Upward movement of the pins onto the PCB edge and subsequently upon the finger upper surface can adversely affect the alignment of one or more pins within the row of pins. Thus, a concern of PCB manufacturers is the possibility of adverse displacement of the compliant pin during times in which the PCB edge enters the slot formed between rows of pins. Significant lateral displacement can cause the pins to reside upon insulative material between fingers rather than on the finger itself. Moreover, upward displacement at abrupt surfaces at the PCB edge can bend or possibly break the pin such that it is incapable of contacting the finger.
If the pin is to be permanently affixed to a finger through, for example, surface mount soldering, then it is important that proper solder connection exists between the pin and respective finger. Permanent surface mount technology (SMT) employs the fingers as bond sites and therefore a more accurate terminology would be to indicate the fingers as "bonding pads". Bonding pads near the PCB edge serve a dissimilar function than component mounting pads placed elsewhere on the PCB. Absent proper placement of pins upon respective bonding pads, subsequent soldering steps (reflow or wave) may not provide adequate connectivity.
It is therefore important that the edge of a PCB be configured to provide channeling of each pin to the center of each respective bonding pad. Channeling must occur with minimal upward and lateral disruption during times in which the PCB edge is forced within the receptor slot. Provided they are properly placed, each pin can then be surface mounted to the pads with a more reliable outcome.